B3

🧠 B3 GCSE Biology — Homeostasis (Top Set Notes)

🏠 What is Homeostasis?

Definition:
Maintaining a stable internal environment despite changes outside the body.

Conditions controlled:

  • Body temperature

  • Blood glucose levels

  • Water & ion levels

Why it matters:
👉 Enzymes only work properly under stable conditions.

🍬 Blood Glucose Control (Higher Tier Detail)Normal blood glucose level

≈ 4–6 mmol/L

The Pancreas — Endocrine Gland

Contains specialised cells:

Cell type

Hormone released

Alpha cells

Glucagon

Beta cells

Insulin

🔁 Negative Feedback ControlWhen blood glucose is HIGH

  1. Beta cells detect rise

  2. Insulin released

  3. Insulin causes:

    • Glucose enters body cells

    • Liver converts glucose → glycogen (glycogenesis)

  4. Level returns to normal

When blood glucose is LOW

  1. Alpha cells detect drop

  2. Glucagon released

  3. Causes:

    • Glycogen → glucose (glycogenolysis)

    • Release into blood

  4. Level rises

💉 Diabetes (Higher Detail)Type 1 Diabetes

Cause:

  • Immune system destroys beta cells

Effects:

  • No insulin produced

  • High blood glucose

Treatment:

  • Insulin injections

  • Carbohydrate control

Type 2 Diabetes

Cause:

  • Insulin resistance

  • Often linked to obesity

Treatment:

  • Diet & exercise

  • Medication

💧 Kidney & Osmoregulation (Higher Tier)

Kidney Functions

  • Remove urea (from amino acid breakdown)

  • Maintain water balance

  • Control ion levels

Structure of the Kidney

From outside → inside:

  1. Cortex

  2. Medulla

  3. Renal pelvis

🧬 Nephron Structure (Higher Detail)

Parts:

  • Glomerulus

  • Bowman’s capsule

  • Proximal tubule

  • Loop of Henle

  • Distal tubule

  • Collecting duct

🚰 Urine Formation — 3 Steps1⃣ Ultrafiltration

Location: Glomerulus

High pressure forces out:

  • Water

  • Urea

  • Glucose

  • Ions

NOT filtered:

  • Proteins

  • Blood cells

2⃣ Selective Reabsorption

Location: Proximal tubule

Reabsorbed:

  • All glucose (active transport)

  • Most water (osmosis)

  • Needed ions

3⃣ Osmoregulation

Controlled by ADH hormone

ADH Control (Higher Tier Process)

If body is dehydrated:

  • Brain detects low water

  • Pituitary releases ADH

  • Collecting duct becomes more permeable

  • More water reabsorbed

  • Small volume of concentrated urine

If hydrated:

  • Less ADH released

  • Dilute urine produced

🌡 Temperature Control (Higher Tier)

Controlled by thermoregulatory centre in the brain.

Too Hot Responses

  • Sweating

  • Vasodilation (more blood to skin)

Too Cold Responses

  • Shivering

  • Vasoconstriction

  • Less sweating

🧪 Key Required Practicals (B3)

Top set must know:

Urine Testing

Test strips used to detect:

  • Glucose

  • Protein

Indicates kidney problems or diabetes.

🧠 Top-Set Keywords You MUST Use in Exams

  • Homeostasis

  • Negative feedback

  • Ultrafiltration

  • Selective reabsorption

  • Osmoregulation

  • Active transport

  • Diffusion

  • Glycogenolysis

⭐ Easy Memory SummaryBlood Glucose

HIGH → insulin → store glucose
LOW → glucagon → release glucose

Kidney

FRO:
Filtration → Reabsorption → Osmoregulation

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